The manufacture of an SOI (Silicon On Insulator or Semiconductor On Insulator) substrate utilizes a porous layer. According to an SOI substrate manufacturing method using a porous layer (see, e.g., Japanese Patent Laid-Open No. 5-21338), for example, a porous layer is formed on a seed substrate, and a semiconductor layer and an insulating layer are formed on the porous layer, fabricating a first substrate. The insulating layer side of the first substrate is bonded to a second substrate (handle substrate), fabricating a bonded substrate. After that, the bonded substrate is split into two substrates at the porous layer. As a result, the insulating layer and semiconductor layer on the seed substrate are transferred onto the handle substrate, obtaining an SOI substrate. This method is known as an ELTRAN method (ELTRAN is a registered trademark).
In the manufacture of an SOI substrate, the porous layer is used to form a semiconductor layer (e.g., an epitaxially grown layer) on it and by splitting a bonded substrate, and thus management of the quality of the porous layer is important. As a method of evaluating the characteristics of a porous material, a gas absorption method is known. However, this method takes a long time for sample preparations, such as degassing and temperature setting, and exhibits low evaluation efficiency.
A technique of measuring the thickness of a light transmitting film is disclosed in Japanese Patent Laid-Open No. 2000-314612. However, the technique in Japanese Patent Laid-Open No. 2000-314612 is irrelevant to the measurement of the quality.